The present invention relates to a transconductance amplifier for converting an input voltage into an output current. Depending on the intended application of the amplifier, the transfer characteristic between the input voltage and the output current may be linear when the circuit is used in an adjacent channel filter and may be square law when used in an automatic gain control circuit based on the sum of the squares of quadrature related signals.
Transconductance amplifiers are known per se from EP-B1-0 234 655, , which corresponds to U.S. Pat. No. 4,723,110 (Feb. 2, 1988). In a simplified form they may be regarded as a long tail pair circuit in which the control electrodes, for example, base electrodes, are interconnected by a resistor. In developments of this simple circuit, the resistor is symmetrically tapped at various points beginning at each end and proceeding to the center. Although tapping the resistor in this way increases the range of input voltages which can be handled, the problem arises that there is a practical limit to the overall value of the resistor because the higher the value of resistance, the greater the error caused by the flow of base currents. This error is small if the resistance has a relatively small value, but this resistance also forms the output load of the preceding stage so its current consumption will be higher compared to the case where the resistance has a higher value. Accordingly, a transconductance amplifier of such a design is less attractive to use in circuits for inclusion in battery powered equipment. FIG. 12 of EP-B-0 234 655 discloses a base current compensation technique which although theoretically perfect can suffer from the drawback of having to work with unacceptably small currents of the order of nanoamps so that the leakage currents of the transistors set the limit.